The present invention pertains generally to laser systems and more particularly to an apparatus and method for initializing an optical-fiber for mode locking by introducing a controllable phase shift into the laser cavity.
Modern communication systems increasingly use light waves traveling through optical fibers to carry information such as telephone conversations, television signals, digital data for computers, and the like. Optical fiber systems need reliable sources of narrow light pulses at high repetition rates. These narrow optical pulses are needed not only for optical communications but also for other kinds of lightwave instruments and more generally in other fields of scientific research.
Narrow optical pulses can be generated by a mode-locked laser. A laser produces a number of optical signals over a band of frequencies. If these signals are in phase, they combine to produce a narrow pulse. The laser is said to be mode-locked when theses signals are synchronized such as to produce a series of narrow pulses at a desired repetition rate.
Semiconductor lasers have been used to generate optical pulses. Although such lasers can produce very narrow pulses at high repetition rates, they generate chirped pulses which are not transform-limited and therefore are undesirable.
The fiber optic FIG.-8 laser (referred to herein as "F8L") shows great promise as a source of narrow short optical pulses. Existing F8Ls can generate pulses having widths of hundreds of femtoseconds and repetition rates of a few gigahertz. These pulses are better than those produced by semiconductor lasers in that they are transform-limited and have a hyperbolic secant squared ("sech.sup.2 ") shaped enveloped in the time domain. Some applications for which the F8L is potentially well-suited include: a source for soliton transmissions, building compact electro-optic sampling systems, testing soliton transmission systems, and a lightwave source for time domain testing of high-speed lightwave systems. Unfortunately, this promising technology encounters some major obstacles.
One major stumbling block is the initiation of mode-locking. A F8L is structured in two loops, one of which, referred to as a Sagnac loop, includes an amplifying medium such as an erbium-doped fiber. An optical pump is used to activate the laser. Theoretically, the only limits to the repetition rate of the short optical pulses generated within the F8L are believed to be those of pulse energy and power extraction from the amplifying medium. However, in real fiber optics systems, birefringence, the phenomenon of light waves having different polarizations travelling at different velocities through a medium, makes it difficult to start and control the mode-locking in the laser.
Attempts have been made to control the effects of birefringence by optimizing the polarization in the F8L. Accordingly, one method of optimization is placing polarization controllers in the F8L. This method is hit or miss and is often too slow. A proposed method is fabricating the Sagnac loop from polarization-preserving fiber. However, erbium doped fiber amplifiers are not available in polarization-preserving form. Thus, no systematic solution to the birefringence problem has been found.
Even in F8Ls using polarization preserving schemes, mode-locking cannot be started consistently. Thus, the desired operating mode is difficult to initialize. Attempts to start the system using excessively strong pumping or mechanical perturbations of the system itself have been unreliable. When mode-locking does occur, the pulses often bunch around a fundamental repetition rate and are not equally paced apart. Further, if mode-locking is successfully initiated, the continued generation of short optical pulses is not guaranteed. Accordingly, it can be seen that the development of a F8L that is easy to start and that reliably produces narrow periodic pulses at a desired repetition rate continues to pose a difficult challenge.
The repetition rates of F8Ls are usually low, in the range of several MHz. For many applications, such as trans-oceanic soliton transmission and high speed electro-optics sampling, it is essential that the F8L produce increased round-trip repetition rates. Attempting to increase the repetition rate by increasing the pumping intensity results in sporadic and uncontrollable pulses.
One method of increasing the round-trip repetition rate is to employ a Fabry-Perot etalon, an interferometer with fixed mirror separation. The Fabry-Perot etalon is placed into the F8L to control the repetition rate through constructive interference. The number of pulses in a given period is always an integer multiple of the number of completed waves during that period. Although use of the Fabry-Perot etalon provides an adequate repetition rate, it makes starting the laser more difficult.
Much of the current research in F8Ls is focused on one or more of the aforementioned problems. As yet, little attention has been paid to another serious problem: timing jitter. Ideally, the output pulses should be evenly spaced in the time domain. Without active stabilization, the output pulses are prone to timing jitter and cannot be locked to a reference signal.
From the foregoing, it will be apparent that there is a great need for a F8L that can be reliably started and is capable of generating short optical pulses at a controllable, high repetition rate, with a minimum of timing jitter.